Search Results

You are looking at 1 - 10 of 14 items for :

  • "hydrogen reduction" x
  • All content x
Clear All

Abstract  

The method of a stepwise differential isothermal analysis (SDIA) has been designed for studies of reduction processes in metallurgical systems. The basis of the method is in multi-parameter control of reaction rate and its use for monitoring temperature, heating rate, and gas-phase composition. In this study hydrogen reduction of MoO3 and its mix with 30% Fe have been carried out using the SDIA technique. During the measurements, TG, DTG, DTA, EGA, and temperature control are carried out. Kinetics parameters were determined and possible reduction mechanism was suggested. The SDIA technique is well suited for these studies.

Restricted access

Abstract  

This paper describes a study on some physico-chemical properties of CuO−Bi2O3 mixed oxides of various composition and their reactivity during hydrogen reduction in the range 290–460°C. Depending on the composition, the changes in the morphology of the samples, their specific surface areas, phase composition of Bi2O3 as well as the change in the amount of chemisorbed surface oxygen (specific surface oxidation ability) were found. The last mentioned parameter is strongly affected positively or negatively (depending on the dose absorbed) by the pre-irradiation with gamma-rays and accelerated electrons. Either reduction of CuO or consecutive reduction of both components with a maximum rate on the surface or sub-surface layers of grain, proceed in different temperature ranges. The retarding effect of liquid bismuth is partially compensated for by the sponging and trapping effects, so that the overall reduction rate changes non-monotonously with the composition of the samples. Pre-irradiation leads in all cases to the lowering of reduction rate, which can be correlated with the increase in the concentration of strongly bound oxygen forms, creating centres of donor hydrogen chemisorption. The effect of gamma-radiation appears to be function of the threshold dose absorbed.

Restricted access

Abstract  

The physico-chemical properties and reactivity tested by hydrogen reduction have been studied for two series of NiO-ZnO mixed oxides of various composition. The solid nickel oxide or zinc oxide in interaction with the solution of nitrate of the second component were used as the precursors in each series. The differences in some physico-chemical parameters of the samples in both series were correlated with their reduction behaviour, followed both in iso- and non-isothermal regime. Moreower, the influence of various factors modifying the reactivity of mixed oxides was also investigated and the results were compared with those obtained from earlier studied analogous systems of quite different origin.

Restricted access

Abstract  

Some physico-chemical properties of NiO–ThO2 mixed oxides of various compositions have been investigated. The presence of strongly bound constitutional water in the hydrogel of reactive forms of thorium dioxide, determined by their origin (thermal decomposition of mixed hydroxides) caused the different reduction behaviour as compared with other mixed oxide systems containing the only, thermodynamically less stabile reducible component. The significant effects of the thermal treatment in oxygen atmosphere, pre-irradiation by the gamma rays or accelerated electrons under various conditions (in air or in water suspension) as well as of surface chemical activation with a platinum complex on the reactivity of mixed oxides or reoxidized samples during their hydrogen reduction have been also proved.

Restricted access

Kinetics of low temperature hydrogen reduction of the metastable spinels

Magnetite and solid solutions with Mn, Co, Ni and Cu

Journal of Thermal Analysis and Calorimetry
Authors: D. Drakshayani and R. Mallya

Abstract  

The kinetics of reduction at relatively low temperatures with hydrogen of pure and doped metastable non-stoichiometric magnetite with 1 at% Mn, Co, Ni and Cu and also with 5 at % Ni and Cu have been investigated by using isothermal thermogravimetry in the temperature range 300–400°C. With increase in the concentration of the dopant (5 at% Ni and Cu), the reactivity increases. The activation energies for pure magnetite varies from 7 to 9 kcal/mole with the preparation temperature of precursorf Fe2O3 (250–400°C), being the lowest for those prepared at the lowest temperatures. The corresponding activation energies for the reduction of doped samples (Fe, M)3−zO4, it depends, apart from their porosity and surface areas, on the nature of the solute atom, amount of disorder, whether it occupies the tetrahedral (A) or octahedral (B) sites in the non-stoichiometric spinel and possibly on hydrogen ‘Spill over’ effects.

Restricted access

Abstract  

Some physico-chemical properties and reactivity in their reduction with hydrogen of NiO—Y2O3 mixed oxides prepared in a dry way have been studied using isothermal thermogravimetry in the range of 320–410°C and temperature-programmed reduction. It was found that addition of small amounts of chloride and acetate anions retarded the reduction of nickel oxide and accelerated the reduction of mixed oxides. The presence of oxalate and formate ions manifests itself by a small positive effect. Introduction of platinum activator or heat treatment of the samples in various atmospheres led to a pronounced increase in the reduction rate. The efficiency of the spill-over effect increases with increasing proportion of non-reducible Y2O3. The pre-irradiation of the samples by accelerated electrons and gamma rays at a dose of 1 MGy results in a negative kinetic effect only with the samples containing an excess of nickel oxide.

Restricted access
Journal of Thermal Analysis and Calorimetry
Authors: T. Hashizume, K. Terayama, T. Shimazaki, H. Itoh, and Y. Okuno

Abstract  

We succeeded in studying the mechanism of hydrogen added carbothermic reduction process of iron-manganese oxide by means of the new technique, simultaneous measurement of evolved gas analysis (EGA) and humidity sensor (HS). Water vapor evolved by the reduction with hydrogen can be detected by HS. Other gas was detected by TCD. Without carbon, the hydrogen reduction process was followed to the formation of the intermediate product between MnO and FeO and finally reduction to the mixture of MnO and Fe. With carbon, the intermediate products between MnO and FeO was formed at about 780 K. The methane was formed in higher temperature than 1073 K and the reduction with carbon proceeded mainly. At higher temperatures, methane decomposed to yield nascent carbon that tended to result in the acceleration of the reduction rate with carbon. The study is concerned with the mechanism of the hydrogen reduction of MnFe2O4 and the effect of without and with carbon on this reduction by means of combining EGA and HS.

Restricted access

Abstract  

Thermodynamic calculations predict the formation of hydrochloric acid gas and alkali tungstates during hydrogen reduction of WO3 doped with alkali chlorides MCl (M=Li, Na, K). The formation of HCl was proved experimentally by simultaneously coupled TG-MS measurements from RT to 1200C. The formation of HCl is the result of the reaction between MCl, WO3 and water. Ubiquitous traces of moisture in the gas are sufficient for reaction according to WO3+(2+2n)MCl +(1+n)H2O→M2+2nWO4+n+(2+2n)HCl (n=0, 1, 2). Laboratory reduction tests showed that the formed tungstates differ. NaCl and KCl form monotungstates (n=0), while LiCl produces more lithium-rich compounds (n=1, 2). Temperature and humidity, among other process factors, control subsequent reduction of the tungstates to metals.

Restricted access

Abstract  

Temperature programmed hydrogen reduction studies have been carried out for SnO2and Ce-Sn mixed oxides with and without Pd metal impregnation, to demonstrate the existence of spillover of hydrogen from Pd metal centers to support oxides. TPR pattern of SnO2 showed a main peak at ~973 K indicating the bulk reduction of this sample. In Pd metal impregnated sample, the bulk reduction peak shifts to lower temperature (~923 K) due to the spillover of activated hydrogen from Pd metal to SnO2 at relatively lower temperatures and its subsequent reaction with SnO2. For Pd impregnated Ce-Sn mixed oxide samples also, a similar effect or an enhanced reduction was observed indicating the spillover effect of hydrogen. These results have been further confirmed from 119Sn Mössbauer spectroscopic measurements carried out for some representative samples of SnO2 and Pd/SnO2 heated in hydrogen flow up to a temperature of 473 K. The value of Sn2+/(Sn4++Sn2+) ratio was found to be significantly higher for Pd impregnated sample. Both these observations provide direct evidence for the existence of spillover effect of hydrogen taking place in the metal impregnated samples.

Restricted access